Method of making a threaded connection for reinforcing bars

ABSTRACT

A process for forming a screw thread on a corrugated bar used in concrete reinforcement joints such as anchor bolt joints. The ribs and flank fillets are removed from the end portion of the corrugated bar to be threaded. The end portion is then heated in a controlled manner such that the temperature of the end portion is highest at the end and falls smoothly toward the beginning of the ribless portion of the bar. The bar is immovably locked in place and the end portion is pressed by a movable cylindrical mold. Due to the temperature difference along the end portion, hot upsetting sets in at the end and advances toward the beginning of the ribless portion and the cross-sectional area of the end portion expands to become equal to an inner diameter of the mold. A screw thread is then formed on the end portion of the bar, the cross-sectional area of the screw thread being at least equal to the cross-sectional area of a remainder of the bar and the thread being longer than a diameter of the bar.

The present invention relates to a procedure for making a screw threadon a corrugated bar.

The corrugated bar used as raw material in the procedure of theinvention for making a screw thread is produced from round section by aspecial forming method which produces the ribs of the corrugaged bar andsimultaneously increases the strength of the steel during themanufacture. This manufacturing method increases the strength of thecorrugated bar, and in addition the ribs produced become hardened, beingof a considerably harder material.

Normal screw threads for a nut on a corrugated bar are made using knowntechniques either by rolling or by cutting. In these cases, thecross-section of the bar is reduced in the threaded portion and thetensile capacity of the bar is completely determined by thecross-section of the thread. The reduction in tensile capacity of thecross-section of the thread as compared to a solid bar is of the orderof 20-30%. Thus, the tensile capacity of a threaded bar is exclusivelydetermined by the cross-section of the thread, leaving the capacity ofthe rest of the bar unused, which means uneconomic use of steel. Aneconomic target is to produce a thread whose tensional area is largerthan or as large as the nominal area of the corrugated bar.

For the manufacture of a screw thread having the full tensile capacityof the corrugated bar, several methods have been patented. These arebased on expanding the end of the corrugated bar by the cold upsettingmethod in room temperature. For example, patent application GB 2 227 802presents a bar joint for use in the reinforcement of concrete, in whichthe cross-section of the bar ends to be joined is enlarged by coldupsetting and the ends are provided with a conical thread.FI-application 890509 presents a procedure for making mechanical jointsbetween round reinforcement bars, in which the bars are joined togetherby means of a threaded sleeve placed at the juncture. According to thisapplication, one or both ends of the bars to be joined are cold upsetbefore threading. The upsetting is performed over the length of the partto be threaded and is so implemented that the root diameter of thethreaded portion is at least equal to or larger than the normal diameterof the bars to be joined.

Cold upsetting causes no changes in the material or strength propertiesof corrugated bars. After the cold upsetting, the thread is produced onthe upset area by cutting. This method preserves the strength propertiesof the steel bar unchanged, but it also removes material from thesurface of the bar. By the cold upsetting method, the end of thecorrugated bar can only be enlarged over a short length because thematerial structure of steel does not withstand cold upsetting wellenough to allow a corrugated bar to be provided with a thread longerthan that required for a nut. For joints requiring a long thread, thecold upsetting method is inadequate.

The object of the present invention is to eliminate the drawbacks ofpreviously known techniques and to achieve a procedure for making athread on a corrugated bar which preserves the increased strength ofsteel achieved during the manufacturing of the corrugated bar as well asthe hardness of the steel surface and the toughness of the interiorparts of the bar even during the threading process, allowing a threadwith a full tensile capacity to be made on the corrugated bar.

In the procedure of the invention, the end of the corrugated bar ismachined by removing the corrugation ribs and flank fillets of the bar.Next, the bar end is heated and then hot upset, thereby increasing itscross-sectional area. After the hot upsetting, the upset end of thecorrugated bar is cooled. The bar end is threaded by rolling. Thedetails of the features characteristic of the procedure of the inventionare presented in the attached claims.

This procedure allows one to produce a thread with a tensionalcross-section as large as or larger than the net cross-sectional area ofa solid corrugated bar, which is decisive in respect of the boltratings. Moreover, regardless of the diameter of the corrugated bar, thethreaded portion can be of a desired length depending on the use it isdesigned for. This means that all of the tensile capacity of thecorrugated bar can be utilized, including the threaded portion, and theprocedure makes it possible to produce a thread of any length asrequired. Thus, a threaded corrugated bar can be used in applicationsrequiring a thread longer than that required by the nut length, in otherwords, the thread can be long enough to allow adjustment as required.Such applications include e.g. the anchor bolt joints of pillars.

In the following, the invention is described in detail by the aid of anexample by referring to the attached drawing, in which

FIG. 1a presents a corrugated bar and FIG. 1b a corrugated bar with amachined end.

FIG. 2 illustrates the hot upsetting procdure.

FIG. 3a presents a hot upset bar end and FIG. 3b a corrugated barprovided with a screw thread according to the invention.

In the procedure for making a full-capacity screw thread, the end of thecorrugated bar is first machined by turning it so as to remove thecorrugation ribs 1 and the flank fillets 2 (FIG. 1a) from the bar area 3to be threaded (FIG. 1b). In this way, the hardest parts of thecorrugated bar are removed. In the manufacturing process of corrugatedbars, the rib material undergoes the greatest changes. In the procedureof the invention, the parts of the hardest material, which constitute animpediment to hot upsetting as employed in the thread-making procedure,are removed from the corrugated bar.

The machined end 3 of the corrugated bar (FIG. 1b) is heated in acontrolled manner so that a smooth temperature difference is created inthe machined area 3 between the bar end 4 and the beginning 5 of theribbed portion, the temperature being highest at the end 4 of thecorrugated bar and falling smoothly towards the other end 5 of themachined portion. The temperature of the unmachined portion 6 of thecorrugated bar is not raised except by heat transfer from the heatedportion 5.

The heated corrugated bar 7 (FIG. 2) is locked in place by means of ahydraulic press 8 so that it cannot move. With another hydraulic press9, a closed cylindrical mould 10 is pressed against the bar end 11 sothat the end 11 of the corrugated bar begins to be hot upset and itscross-sectional area increases and becomes equal to the internaldiameter of the cylindrical mold 10 in the press.

The end 7 of the corrugated bar is expanded so much that thecross-sectional area of the thread 15 to be formed will be at leastequal to the cross-section of the rest of the bar 7, so that the tensilecapacity of the bar is fully preserved even in the threaded portion.

The pressing force is applied from the end 11 of the bar towards thelocking part 8 and is large enough to upset the bar and increase itscross-sectional area to the size of the mold. The purpose of thechanging distribution of temperature in the machined portion of the baris to ensure that the hot battering effect will start from the end 11 ofthe bar and, as the pressing force is increased, advance towards theother end 12 of the machined portion. With the smoothly changingtemperature, the advance of the upsetting of the bar can be controlledall the time, and it also ensures that the upsetting will not start atthe middle of the machined portion. Moreover, the temperature risingtowards the end 11 of the bar ensures that the portion to be upset willnot buckle before the upsetting effect sets in at the hottest point 11 .The molding is only stopped after the whole machined portion 13 hasexpanded and fills the mold 10.

After the hot upsetting, the upset end 14 of the bar (FIG. 3a) is cooledin a controlled manner so that the original strength characteristics ofthe corrugated bar can be preserved during the cooling process.

To make a full-capacity screw thread, the rolling method as known inprior art is used, whereby the cylindrical portion 14 formed on the barvia hot upsetting is worked with rollers to form a screw thread on theupset end of the bar without removing any material from it.

Through the rolling process, a thread is formed on the surface of thebar, and the rolling also has a strengthening effect on the material asthe steel material 17 under rolling is cold formed, thereby increasingits strength and hardness. The cold strengthening effect of the rollingdoes not reach the interior part 18 of the bar, so the material insidethe bar remains tough and the toughness characteristics of the wholethreaded portion of the bar are preserved.

The rolling for the forming of the thread is only started after the endof the corrugated bar has been cooled to room temperature. The thread ismade on the whole upset portion 14 of the corrugated bar. After this, nomore turning is done on the bar.

By using the rolling method, the original hardness of the material inthe threaded portion, which was lost during heating, is restored. Inaddition, the rolling also causes the bar material to becold-strengthened in the threaded portion, enabling the originalhardness of the surface of the corrugated bar to be restored in thispart of the bar. The cold strengthening effect of the rolling does notreach the interior parts of the bar, so the good toughness properties ofthe corrugated bar can be preserved even in the threaded portion.

It is obvious to a person skilled in the art that different embodimentsof the invention are not restricted to the example described above, butthat they may instead be varied within the scope of the followingclaims.

I claim:
 1. A process for forming a screw thread on a corrugated bar,said process comprising the steps of:removing ribs and flank filletsfrom an end portion of a corrugated bar to be threaded, after removalsaid end portion extending from an end of said bar to a beginning of aribless portion of said bar; heating said end portion of said bar in acontrolled manner by starting heating at said end such that thetemperature of said end portion is highest at said end and fallssmoothly toward said beginning of said ribless portion; immovablylocking said bar in place; pressing said end portion of said bar bymeans of a movable cylindrical mold, the temperature difference betweensaid end and said beginning of said ribless portion causing hotupsetting to set in at said end and advance toward said beginning ofsaid ribless portion, whereby the cross-sectional area of said endportion of said bar expands to become equal to an inner diameter of saidmold; and forming a screw thread on said end portion of said bar, thecross-sectional area of said screw thread being at least equal to thecross-sectional area of a remainder of said bar and said screw threadbeing longer than a diameter of said bar.
 2. The process of claim 1,wherein the screw thread is formed by rolling.
 3. The process of claim2, wherein the corrugated bar is used in a concrete reinforcement joint.4. The process of claim 2, wherein the corrugated bar is used in ananchor bolt joint.